1. Field of the Invention
The invention relates to the preparation of synthetic lubricant base stocks, and more particularly to synthetic lubricant base stocks made by oligomerizing linear olefins.
2. Description of Related Methods
Synthetic lubricants are prepared from man-made base stocks having uniform molecular structures and, therefore, well-defined properties that can be tailored to specific applications. Mineral oil base stocks, on the other hand, are prepared from crude oil and consist of complex mixtures of naturally occurring hydrocarbons. The higher degree of uniformity found in synthetic lubricants generally results in superior performance properties. For example, synthetic lubricants are characterized by excellent thermal stability. As automobile engines are reduced in size to save weight and fuel, they run at higher temperatures, therefore requiring a more thermally stable oil. Because lubricants made from synthetic base stocks have such properties as excellent oxidative/thermal stability, very low volatility, and good viscosity indices over a wide range of temperatures, they offer better lubrication and permit longer drain intervals, with less oil vaporization loss between oil changes.
Synthetic base stocks may be prepared by oligomerizing internal and alpha-olefin monomers to form a mixture of dimers, trimers, tetramers, and pentamers, with minimal amounts of higher oligomers. The unsaturated oligomer products are then hydrogenated to improve their oxidative stability. The resulting synthetic base stocks have uniform isoparaffinic hydrocarbon structures similar to high quality paraffinic mineral base stocks, but have the superior properties mentioned due to their higher degree of uniformity.
Synthetic base stocks are produced in a broad range of viscosity grades. It is common practice to classify the base stocks by their viscosities, measured in centistokes (cSt) at 100.degree. C. Those base stocks with viscosities less than or equal to about 4 cSt are commonly referred to as "low viscosity" base stocks, whereas base stocks having a viscosity in the range of around 40 to 100 cSt are commonly referred to as "high viscosity" base stocks. Base stocks having a viscosity of about 4 to about 8 cSt are referred to as "medium viscosity" base stocks. The low viscosity base stocks generally are recommended for low temperature applications. Higher temperature applications, such as motor oils, automatic transmission fluids, turbine lubricants, and other industrial lubricants, generally require higher viscosities, such as those provided by medium viscosity base stocks (i.e. 4 to 8 cSt grades). High viscosity base stocks are used in gear oils and as blending stocks.
The viscosity of the base stocks is determined by the length of the oligomer molecules formed during the oligomerization reaction. The degree of oligomerization is affected by the catalyst and reaction conditions employed during the oligomerization reaction. The length of the carbon chain of the monomer starting material also has a direct influence on the properties of the oligomer products. Fluids prepared from short-chain monomers tend to have low pour points and moderately low viscosity indices, whereas fluids prepared from long-chain monomers tend to have moderately low pour points and higher viscosity indices. Oligomers prepared from long-chain monomers generally are more suitable than those prepared from shorter-chain monomers for use as medium viscosity synthetic lubricant base stocks.
One known approach to oligomerizing long-chain olefins to prepare synthetic lubricant base stocks is to contact the olefin with boron trifluoride together with a promotor at a reaction temperature sufficient to effect oligomerization of the olefin. See, for example, co-assigned U.S. Pat. Nos. 4,400,565; 4,420,646; 4,420,647; and 4,434,308. However, boron trifluoride gas (BF.sub.3) is a pulmonary irritant, and breathing the gas or fumes formed by hydration of the gas with atmospheric moisture poses hazards preferably avoided. Additionally, the disposal/neutralization of BF.sub.3 raises environmental concerns. Thus, a method for oligomerizing long-chain olefins using a non-hazardous, non-polluting catalyst would be a substantial improvement in the art.
Kuliev et al. attempted to prepare synthetic lubricants by oligomerizing long-chain (C.sub.9 -C.sub.14) olefins using non-hazardous and non-polluting acidic clays comprising sulfuric and hydrochloric acid-activated bentonites from the Azerbaidzhan SSR. See Kuliev, Abasova, Gasanova, Kotlyarevskaya, and Valiev, "Preparation of High-Viscosity Synthetic Lubricants Using an Aluminosilicate Catalyst," Institute of Petrochemical Processes of the Academy of Sciences of the Azerbaidzhan SSR, Azer. Neft. Khoz., 1983, No. 4, pages 40-43. However, Kuliev et al. concluded that "it was not possible to prepare viscous or high-viscosity oils by olefin polymerization over an aluminosilicate catalyst" and that "hydrogen redistribution reactions predominate with formation of aromatic hydrocarbon, coke, and paraffinic hydrocarbon." Gregory et al., on the other hand, used Wyoming bentonite to oligomerize shorter-chain olefins. (See U.S. Pat. No. 4,531,014.) However, like Kuliev et al., they also were unable to obtain a product high in dimer, trimer and tetramer, and low in disproportionation products.
Applicants have discovered, surprisingly, that a high conversion of olefin to oligomer may be obtained by contacting the olefin with a catalyst prepared by depositing a halogenated phosphorous-containing acid on a substrate comprising montmorillonite clay. Applicants have further discovered that where the halogenated phosphorous-containing acid is a fluorinated phosphorous-containing acid, the process of the present invention results in a very high percentage of trimer and higher oligomers, i.e., a very low dimer to trimer ratio. A high proportion of trimer and higher oligomers is particularly desirable when preparing a synthetic lubricant base stock from decene. In the absence of the low dimer to trimer ratio obtained using the present invention, substantial decene dimer must be recycled and further oligomerized to prepare enough oligomers having sufficient molecular weight to obtain base stocks suitable for synthetic lubricants. In addition to being excellent catalysts, the treated clays of the present invention are less hazardous and more easily handled than BF.sub.3.